Since the Hawaiian Islands have never been connected to other land masses, the great variety of plants in Hawaii must be a result of the long-distance dispersal of seeds, a process that requires both a method of transport and an equivalence between the ecology of the source area and that of the recipient area.

There is some dispute about the method of transport involved. Some biologists argue that ocean and air currents are responsible for the transport of plant seeds to Hawaii. Yet the results of flotation experiments and the low temperatures of air currents cast doubt on these hypotheses. More probable is bird transport, either externally, by accidental attachment of the seeds to feathers, or internally, by the swallowing of fruit and subsequent excretion of the seeds. While it is likely that fewer varieties of plant seeds have reached Hawaii externally than internally, more varieties are known to be adapted to external than to internal transport.

The author mentions the results of flotation experiments on plant seeds most probably in order to

(A) support the claim that the distribution of plants in Hawaii is the result of the long-distance dispersal of seeds

(B) lend credibility to the thesis that air currents provide a method of transport for plant seeds to Hawaii

(C) suggest that the long-distance dispersal of seeds is a process that requires long periods of time

(D) challenge the claim that ocean currents are responsible for the transport of plant seeds to Hawaii

(E) refute the claim that Hawaiian flora evolved independently from flora in other parts of the world

I thought the correct answer is D, but the correct answer is C. Would you please explain the reason behind it?